In general, a mode locking technique is a light source technique for creating a short optical pulse by applying a periodic electrical signal having a cycle calculated by dividing a round-trip time of a photon in a resonator by an integer multiple.
The intensity of an output light pulse oscillated in this case is maximized when a mode locking condition that a cycle of an electrical signal should correspond to an integer multiple of a round-trip time of a photon is satisfied. Generally, such a light source is applicable to optical communication technology or an image measuring device configured as a separate device.
An image measuring device using mode-locked laser includes an optical coherence tomography (OCT) imaging device which obtains a depth-direction image of a sample using a light coherence phenomenon.
The OCT imaging device is an imaging system capable of obtaining and displaying an image of a cross section of an internal tissue of a sample. The OCT imaging device is a device employing an interference principle of a light source having a wavelength range of near-infrared light using an additional optical interferometer.
In particular, an OCT imaging technique is an imaging technique for contrasting the inside of a sample in a non-contact manner. Recently, research has been being actively conducted thereon.
In the OCT imaging device, center wavelength-variable laser is used to obtain depth-direction information. In the wavelength-variable laser using mode locking, a cycle of a periodic electrical signal is changed over time using a dispersion medium or a wavelength light path difference induction device. Thus, a wavelength of light is changed using output light oscillating while satisfying a different mode locking condition for each wavelength. In this case, the cycle of the periodic electrical signal is linearly changed over time and thus the intensity of the output light is constantly maintained while a wavelength thereof is linearly changed.
Various features, such as a high light output, a wide wavelength-variable range, high wavelength-variable linearity, a narrow oscillation line width, etc., should be satisfied so that the wavelength-variable laser may be applicable to imaging devices.
A maximum image depth or a surface step difference of an optical tomographic imaging device using the wavelength-variable laser does not exceed several mm and is determined by a coherence length of a light source.
Such an OCT imaging device using the wavelength-variable laser includes a wavelength-variable light source unit, a distance difference optical interference unit, an optical interference measuring unit, a time-distance conversion signal processor, etc. as illustrated in FIG. 1, and employs an additional signal processing method to obtain distance information by measuring an interference signal of light which changes over time.
The signal processor performs various signal processing processes such as correcting linearity according to a temporal change, time-distance Fourier transformation, etc. and thus has disadvantages such as time restrictions, an increase in manufacturing costs, etc.
In particular, a range of depth measurement is limited by a coherence length of a light source and thus applying the range of depth measurement to a shape measuring device is limited.